The invention relates generally to electrophotographic printing, and more particularly, a cleaning blade used therein to remove particles adhering to the photoconductive member.
In the process of electrophotographic printing, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is imagewise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. This records an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document. Thereafter, a developer material is transported into contact with the electrostatic latent image. Toner particles are attacted from the carrier granules of the developer material onto the latent image. The resultant toner powder image is then transferred from the photoconductive surface to a sheet of support material and permanently affixed thereto.
This process is well known and useful for light lens copying from an original and printing applications from electronically generated or stored originals, and in ionography.
In a reproduction process of the type as described above, it is inevitable that some residual toner will remain on the photoconductive surface after the toner image has been transferred to the sheet of support material (e.g. paper). It has been found that with such a process that the forces holding some of the toner particles to the imaging surface are stronger than the transfer forces and, therefore, some of the particles remain on the surface after transfer of the toner image. In addition to the residual toner, other particles, such as paper debris (i.e. Kaolin, fibers, clay), additives and plastic, are left behind on the surface after image transfer. (Hereinafter, the term "residual particles" encompasses residual toner and other residual particles remaining after image transfer.) The residual particles adhere firmly to the surface and must be removed prior to the next printing cycle to avoid its interfering with recording a new latent image thereon.
Various methods and apparatus may be used for removing residual particles from the photoconductive imaging surface. Hereinbefore, a cleaning brush, a cleaning web, and a cleaning blade have been used. Both cleaning brushes and cleaning webs operate by wiping the surface so as to affect transfer of the residual particles from the imaging surface thereon. After prolonged usage, however, both of these types of cleaning devices become contaminated with toner and must be replaced. This requires discarding the dirty cleaning devices. In high-speed machines this practice has proven not only to be wasteful but also expensive.
The shortcomings of the brush and web made way for another now prevalent form of cleaning known and disclosed in the art--blade cleaning. Blade cleaning involves a blade, normally made of a rubberlike material (e.g. polyurethane) which is dragged or wiped across the surface to remove the residual particles from the surface. Blade cleaning is a highly desirable method, compared to other methods, for removing residual particles due to its simple, inexpensive structure. However, there are certain deficiencies in blade cleaning, which are primarily a result of the frictional sealing contact that must occur between the blade and the surface.
Dynamic friction is the force that resists relative motion between two bodies that come into contact with each other while having separate motion. This friction between the blade edge and the surface causes wearing away of the blade edge, and damages the blade's contact with the surface. For purposes of this application, volume wear (W) is proportional to the load (F) multiplied by the distance (D) traveled. Thus, W .varies. FD .varies. FVT, or introducing a factor of proportionality K, W=KFVT where K is the wear factor, V is the velocity and T is the elapsed time. Hence, wear increases with larger values of K. Various blade lubricating materials or toner lubricant additives have been proposed to reduce friction which would thereby reduce wear. However, lubricants tend to change the operational characteristics of the printing machine undesirably. For example, a polyurethane blade with a good lubricant in the toner can ideally achieve a frictional coefficient of about 0.5, however, this rarely occurs because of the delicate balance involved in achieving the proper weight percent of lubricant in the toner. (Normal frictional coefficient values for cleaning blades removing toner off of the imaging surface ranges from a low of about 0.5 to a high of about 1.5).
In addition to the problem of wear, blades are also subject to unpredictable failures. In normal operational configuration, with a coefficient of dynamic friction in the range of about 0.5 to about 1.5, a blade cleaning edge or tip in sealing contact with the surface is deformed or tucked slightly. The blade is not in intimate contact with the surface, but slides on toner particles and lubricant to maintain the sealing contact required for cleaning. In this configuration, the blade may flatten particles that pass under the blade and cause impaction of particles on the surface. This process is called cometing because of the comet-like impressions created by the flattened particles. The impact from carrier beads remaining on the surface subsequent to development may damage the blade. Sudden localized increases in friction between the blade and surface may cause the phenomenon of tucking, where the blade cleaning edge becomes folded underneath the blade, losing the frictional sealing relationship required for blade cleaning.
Cleaning blades will eventually wear out due to the effects of abrasion against the surface being cleaned. However, it has been observed that many blades fail well before abrasion has caused appreciable wear of the blade edge. The observed failure rates for cleaning blades in electrostatographic machines show that an appreciable percentage of the failures occur at random intervals. It has also been observed that small damaged areas on the blade edge can grow in size over time, often leading to leakage of toner past the blade in the form of a streak, leading to cleaning failure. The present invention reduces blade failuresassociated with randomly occurring defects to the blade edge.
Blade damage can be caused by collision with developer beads, or by edge defects that can originate in cutting during blade manufacture, or as a result of attempts to clean the blade by wiping it laterally along the edge, thereby producing small tears in the edge. When the damage area is of the order of ten times the diameter of the toner in size, an active leak of toner through the cleaning blade will occur, causing a cleaning failure. Since developer beads are typically about ten times the size of toners, this scale of blade damage can occur frequently due to collisions with free developer beads. Also, it is well known that small defects can propagate, or zip open, in resilient materials such as those used for cleaning blades. These small defects are produced in the cutting operation, or in attempts to clean the blade, or may even result from inhomogeneities in the bulk material prior to cutting. A large number of blades must be replaced as a result of defect propagation, thus it is an object of this invention to eliminate this defect propagation.
The following disclosures may be relevant to various aspects of the present invention and may be briefly summarized as follows:
U.S. Pat. No. 4,770,929 to Nobumasa et al. describes a light weight composite material having a laminated structure comprising 1) a porous fiber layer constructed of reinforcing short fibers which are randomly distributed, 2) a fiber reinforced plastic layer, and 3) a matrix resin.
U.S. Pat. No. 4,778,716 to Thorfinnson et al. describes microfibers which are used to prepare composites having improved impact resistance without a loss in strength and modulus.
U.S. Pat. No. 4,823,161 to Yamada et al. describes a cleaning blade comprising a double-layer structure and a contact member made of a poly(urethane)ureamide polymer held in contact with a toner image bearing member.
U.S. Pat. No. 4,825,249 to Oki et al. describes a sharp, resilient cleaning blade for a photoelectronic copy machine comprising a substrate of urethane rubber coated with perfluoropolyether.
U.S. Pat. No. 4,978,999 to Frankel et al. describes a cleaning blade that incorporates fiber fillers that are oriented in a single direction in an elastomeric matrix.